1. Field of the Invention
This invention relates to power supplies and, more particularly, to ringing signal power supplies suitable for telephone networks.
2. Description of the Prior Art
Telephone switching systems require the ringing signals emanating from the central office to comprise a DC voltage on which there is superimposed a 20 Hz AC voltage whose magnitude is larger than the magnitude of the DC voltage. These ringing signals are generally obtained by superimposing the regulated output of a 20 Hz ringing inverter or generator on the central office DC battery. The DC voltage control is obtained by inserting in series with the battery a string of varistor diodes (of appropriate number) which serve as voltage reducing elements. To insure that only the DC voltage is reduced by the diode string, the string is bypassed with a large capacitor. Although the prior art regulation method yields a simple and reliable circuit, the resulting DC regulation is poor. Tighter regulation is highly desirable, in order to avoid both failure to trip the ring relay in the central office--in long subscriber loops, and false tripping--in short subscriber loops. In the No. 1 ESS network, the required ringing signal is 86 volts AC superimposed on 39.+-.1.0 volts DC. Such tight voltage regulation cannot be obtained by employing the diode string voltage reducing method.
In the voltage regulation art, it is well known that DC voltages can be regulated with series or shunt DC regulators. These regulators, however, develop only DC output voltages and, when designed for positive sources, cannot respond to AC sources that are negative at any time.
It is an objective of this invention to employ the teachings of the regulator art and to develop therewith a ringing voltage generator that would be capable of handling source voltages that vary between negative and positive potentials, that would provide the desired DC voltage with the superimposed AC voltage, and that would do so without the use of power conveying transformers.